Solar cells provide widespread benefits to society by converting solar energy into useable electrical power. Demand for higher efficiency solar cells continues to motivate development of new methods of manufacturing solar cells.
In a typical solar cell, solar radiation illuminates one surface of a solar cell, usually referred to as the front side or the sunny side. In many solar cells, a reflective layer is provided over the back side to improve internal light entrapment. One factor that may affect the efficiency of a solar cell is shading of the front surface by adjacent fixtures or by portions of the solar cell, such as shading from metal electrodes. In general, an optimized metal electrode grid of a solar cell requires balancing losses between shading of the solar cell surface by the electrodes and the electrical resistance of the metal structure. Optimizing the efficiency of the solar cell generally requires forming a metal electrode with a pattern of narrow electrically conductive “fingers” with short distances therebetween.
Current solar cell production methods may use varying methods for forming the metal structures and electrodes. For example, a silver paste may be printed over an anti-reflective coating layer, such as a silicon nitride coating, formed on a surface of a solar cell structure, and then fired through the anti-reflective coating in a high-temperature process. However, such processes may result in conductive patterns with a relatively wide metal finger having a width in excess of 50 μam (typically about 80 μm). The processes may also result in lower conductivity of the metal grid pattern due to the use of several non-metallic components in the silver paste. The firing processes may also result in a penetration of the metal paste components through the anti-reflective layer into the substrate of the solar cell structure, whereby increased recombination may occur. This may undesirably affect the p-n junction in front-junction solar cell devices, or may reduce the collection efficiency of back-junction solar cell devices.
One possible method of forming metal structures and electrodes is depicted in FIGS. 1A-1D. FIG. 1A depicts the beginning of the process, which includes using photo-sensitive resist layers 130 deposited over an anti-reflective coating layer 120 over a solar cell structure 110. In FIG. 1B, the resist layer 130 is partially exposed to ultra-violet light to form the desired pattern, followed by etching a portion of the anti-reflective coating 120 through the exposed portions of the resist layer 130, usually with an acid solution. Ideally, this etching process forms negatively inclined flanks 140 in the photo-resist layer 120, and exposes a portion of the surface of the solar cell structure 110. In FIG. 1C, a thin metal film 150 may be deposited, generally by evaporation or sputtering, over the surface of the photo-resist layer 130 and the exposed surface of the solar cell structure 110. The negative inclined flanks 140 ensure that the metal film 150 formed over the surface of the solar cell structure 110 is not in contact with the metal film 150 formed over photo-resist layer 130. This permits a lift-off step, in which the photo-resist layer 130, through the uncovered sides of the negative flanks 140, is exposed to a caustic substance 160 that dissolves the resist layer 130. This results in the metal film 150 formed over the photo-resist layer 130 being removed therewith, as best shown in FIG. 1D. Once the metal film 150 formed over the photo-resist layer 130 is removed, only the conductive metal contact 150 over the surface of the solar cell structure 110 remains.
This method generally depends on the formation of the negatively-inclined flanks 140 of the photo-resist layer 130. In some cases, as shown in FIG. 1E, non-ideal vertical flanks 170 may be formed, or, as shown in FIG. 1F, non-ideal positively-inclined flanks 180. These cases result in a continuous metal film layer 150 being formed over the photo-resist layer 130 and the exposed portion of the solar cell structure 110 (as depicted in FIG. 1G for the vertical flanks 170 and FIG. 1H for the positively-inclined flanks 180). Continuous metal film layers 150 make it difficult to uniformly begin the stripping process of the photo-resist layer 130 from the exposed portions of the flanks, as was the case for the negatively-inclined flanks 140. When there is a vertical flank 170 or a positively-inclined flank the metal film 150 prevents the caustic substance from contacting the photo-resist layer 130. This may have undesirable effects on the metal film layer 150 which may increase the processing time for the lift-off process.